By way of a non-limiting example reference is made below to an electric drive unit comprising a brushless three-phase motor with permanent magnets generating a counter-electromotive force (CEMF) with a sinusoidal form for driving solenoid valves and pumps.
Solenoid valve and electric pump applications require minimum acoustic noise and the reduction of both energy consumption and costs.
These needs have led to the adoption of CEMF sinusoidal brushless motors (AC brushless motors) driven by inverters which are able to impart sinusoidal winding currents, rendering obsolete the use of brushless DC motors driven in PWM six-step.
The sinusoidal trend of the CEMF and, together, of the relative phase current, ensure minimum active torque ripple (virtually zero) and consequently low mechanical vibrations and, therefore, acoustic emissions.
It is also possible to minimise current absorption for the generation of a certain drive torque, and therefore maximise the efficiency of the electromechanical conversion, by an optimum driving of the AC brushless motors which are normally driven by current-controlled, impressed voltage inverters.
This drive requires that the switching of the static switches is performed in such a way as to ensure, instant by instant, that the polar axis of the rotor magnetic field remains at 90 electrical degrees relative to the polar axis of the magnetic field generated by the currents circulating in the stator windings, whatever the torque and the rotation speed.
To obtain continuous information regarding the angular position of the rotor, costly sensors are usually used such as, for example, absolute encoders, resolvers or Hall-effect sensors.
The output signals generated by the sensors are then conveniently processed to control the static switches of the inverter so as to maintain the angular shift of 90 electrical degrees between rotor and stator magnetic fields.
The presence of the position sensors renders the operation relatively costly and, therefore, various drive strategies have been developed which do not use them, in jargon “sensorless”, precisely to reduce the costs of the operations.
Amongst these strategies those based on the orientation of the stator and rotor fields (in jargon FOC) guarantee the aforementioned orthogonal relationship of the fields making use of sophisticated and costly integrated circuits (IC) with high calculation capacities (in jargon DSP) performing the real-time calculation of the rotor angular position, based exclusively on electrical quantities (voltages at the motor terminals and currents circulating in the windings) provided by suitable conditioning circuits.
When the “dynamics” of the machine driven are not too intense—and this is the case with electric fans and electric pumps—it is possible to apply an optimum criteria, derived directly from the fundamental mentioned above and described below (the polar axis of the rotor magnetic field is maintained, instant by instant, at 90 electrical degrees to the polar axis of the magnetic field generated by the currents circulating in the stator windings): the drive works in such a way that the CEMF and the phase current are kept in phase; naturally, the aforesaid criterion is complied with at every point of the operational field (torque, rotation speed, D.C. supply voltage).
The “sensorless” drives which implement drive strategies teased on the aforesaid criterion are founded on the reading of electrical quantities (such as voltages at the motor terminals, currents circulating in the motor windings) with the purpose of:                detecting the zero crossing of the CEMF and the current;        assessing the relative phase between CEMF and current;        implementing, lastly, suitable methods of driving the static switches of the inverter which keep in phase the two quantities just mentioned.        
A first drawback of these strategies lies in the fact that for detecting the zero crossing of the CEMF, that is, to read the sign of the CEMF, as soon as the current crossing the windings becomes zero it must be kept as such for a sufficiently long period of time to allow the reading of the CEMF, which is in contrast with the desired sinusoidal trend of the current for obtaining a low acoustic noise.
A solution to this problem has been proposed in patent EP2195916 in the name of the same Applicant. However, the solution identified introduces an incremental cost due to the use of an “analogue” hardware network of the impedance of a phase of the motor.
A second drawback of the aforesaid control strategies is linked to the need to read the phase current. There are basically two approaches for this reading, both costly, according to the state of the art.
A first approach uses at least one IC device which integrates a Hall-effect sensor sensitive to the magnetic field generated by the phase current (solution with galvanic insulation) whilst a second approach uses at least one IC device which integrates an amplifier to process the voltage across an “outside earth” shunt through which the phase current flows (solution without galvanic insulation).
In the first case the IC device must be positioned close to one of the conductors crossed by the phase current and must have a very low sensitivity to the “parasitic” magnetic fields.
In the second case the common-mode input voltage which the amplifier must accept without damaging itself must be at least equal to the supply voltage of the inverter (Vbus).